Embryonic muscle development in rainbow trout (Oncorhynchus mykiss): A scanning electron microscopy and immunohistological study

Embryonic muscle development was studied in rainbow trout (Oncorhynchus myk iss) at low and high temperature using scanning electron microscopy (SEM) a nd immunohistology. Somite development was described starting at stage 16 ( Vernier JM. 1969. AM Embryol Morphogen 4:495-520) for both temperatures, wi th special interest in their shape and size. Muscle differentiation, associ ated with somite growth, is characterized by a larger increase in height co mpared to width and by acquisition of a chevron shape. Thin structures such as striation, sarcomeres, and myofibrils within muscle cells and myotubes were observed starting at the eyed stage (stage 24). Immunohistological ana lyses showed appearance of embryonic fast myosin at stage 20 in the deep pa rt of the somite. The area where myosin was expressed extended in the somit e throughout embryonic development and the presence of myosin was observed in the entire somite at hatching (stage 30). Slow myosin was expressed in a monolayer of superficial cells at the eyed stage and during the entire emb ryonic development. Then it was expressed in a few layers of cells located in the red muscle area. These results suggest that muscle differentiation, characterized by myosin expression, is engaged at stage 20. Myogenesis star ts in the deep part of the somite, near the notochord and progresses latera lly to cover the complete somite at hatching when the somite is composed of muscle fibres exhibiting a high degree of maturity. No significant differe nce was observed in terms of muscular development between low- and high-tem perature conditions. (C) 2000 Wiley-Liss, Inc.